1. Field of the Invention
The present invention awaits to analog video signal circuits, and in particular, to analog video signal circuits for converting color component video signals.
2. Description of the Related Art
As is well known in the art, color video signals are typically generated as three component video signals representing the three primary colors; red (R), green (G) and blue (B). The entire color spectrum can be represented by varying the relative intensities of these three colors. Similarly, color display devices typically, at least internally, require these three signals R, G, B for driving the actual display device, such as a cathode ray tube (CRT), liquid crystal display (LCD), digital light processor (DLP) and others. Accordingly, it would seem logical to convey these three color signals R, G, B from the signal source to the display device, e.g. via a VHF/UHF broadcast, cable or satellite signal, or a color signal storage medium such as video tape or digital video disc (DVD).
However, such color signals R, G, B, often referred to as a single component video signal RGB have two disadvantages associated with them. First, such signals have very high bandwidth, which can be particularly problematic in a broadcast environment. Second, the black and white, or luminance, picture information is combined within the color signals. These disadvantages are typically addressed by converting the original RGB signal into another type of color component signal which is often referred to as a YPbPr signal. As is well known, the Y component of this signal is the black and white picture information, also known as luminance, contained within the original RGB signal. The Pb and Pr signals are color difference signals which are mathematically derived from the original RGB signal.
All of these signals have relative values defined by coefficients established according to the EIA/CEA 770.2-C specification, entitled “Standard Definition TV Analog Component Video Interface”. This standard defines the physical characteristics of an interface and the parameters of the signals carried across that interface using three parallel channels for the interconnection of equipment operating with analog component video signals. Such signals and their coefficients can be represented by the following equations.Y=0.299R+0.587G+0.114B B−Y=−0.299R−0.587G+0.886B R−Y=0.201R−0.587G−0.114B Pb=(B−Y)/1.772Pr=(R−Y)/1.402
Referring to FIG. 1, using the foregoing equations, a straightforward component video signal conversion circuit for decoding the Y, Pb and Pr signals into the corresponding R, G and B signals can be implemented as shown. Amplifier A1 multiplies the Pb signal by the normalized factor of 1.772 to produce the normalized color difference B−Y signal to which the original Y signal is added, thereby producing the B signal. Similarly, amplifier A2 multiplies the Pr signal by the normalized factor of 1.402 to produce the red color difference signal R−Y to which the Y signal is added, thereby producing the R signal. For the G signal, the recreated B and R signals are multiplied in amplifiers A3 and A4 by the normalized factors of 0.194 and 0.509, respectively. The signals produced by amplifiers A3 and A4 are subtracted from the Y signal to produce the G signal.
For purposes of converting the Y, Pb and Pr component signals back to the original R, G and B component signals, these equations can be simplified to the following equations.R=Y+1.402Pr G=Y−0.344Pb−0.714Pr B=Y+1.772Pb 
While the conversion circuitry of FIG. 1 is simple, when implemented in integrated circuit (IC) form, particularly circuits which use complementary metal oxide semiconductor (CMOS) transistors and processes, the absolute values of the resistors and capacitors needed to form such circuitry, particularly the amplifiers, can experience changes by as much as +/−20%. Accordingly, using the circuitry of FIG. 1 as an example, while it may be possible to maintain reasonably accurate conversion results for the R and B signals, the accuracy of the conversion process for the G signal, which has more complicated signal paths for the various signals used to generate the G signal, will likely experience significant conversion degradation, particularly over variations in PVT, i.e., fabrication processes (P), power supply voltage (V), and operating temperature (T).